1. Field of the Invention
The present invention relates to an image input device and particularly to an image input device in which an image sensor output is not affected by fluctuations in frequency when a rare gas discharge lamp is turned on.
2. Description of the Related Art
Conventionally, various apparatuses using an image input device, particularly a color image input device have been produced, for example, a color copying machine, which uses a combination of a color image input device and a laser beam printer (LBP), and a film scanner. Generally, these apparatuses are required to process a large amount of image data at high speed.
In the image input device, a photoelectric converting unit, which sheds light on an object and converts light reflected at the object into an electrical signal, includes a light source for shedding light on the object, an optical system for focusing the reflected light from the object, and image sensors for receiving the light focused and converting it into an electrical signal.
The image input device uses one-dimensional image sensors and two-dimensional image sensors according to the shape of the object, speed, and resolution, and also uses various transferring mechanisms for transferring the object. In the color image input device, the optical system uses color filters for color separation.
In order to process images at high speed, the image sensors constituting the image input device must be operated at high speed. An output I(xcex) of each image sensor varies with the illuminance of light shed on an object, namely, an output L(xcex) of a light source, the reflectance K of an object, the brightness (transmittance) U of a lens, the accumulating time T of each image sensor, the sensitivity S(xcex) of a wavelength of each image sensor, and the transmittance F(xcex) of each color separation filter. Strictly speaking, the reflectance K of an object varies with the wavelength of light. However, this fact does not affect the spirit of the present invention, so the reflectance K of an object and the brightness U of a lens are set to respective constant values independently of the wavelength of light. For the purpose of simplification, the description hereinbelow will be made on the assumption that the reflectance K of an object and the brightness U of a lens are each set to 1 and that the amount P(xcex) of light incident on each image sensor is proportional to the output L(xcex) of a light source.
For example, a xenon lamp, which is one kind of rare gas discharge lamp and affects the incident light amount P(xcex), is used in the color image input device because it generates an output L(xcex) which has rumination characteristics similar to the wavelength characteristics of natural daylight, and which has high luminance. The spectral sensitivity of a CCD line sensor, which is generally used as a color image sensor and which affects the sensitivity S(xcex) of an image sensor, is low for short-wavelength light. As to the characteristics of optical glass filters, which are generally used as color separation filters affecting the transmittance F(xcex), generally speaking, a blue-base optical glass filter has a high transmittance and gentle characteristics, and gets mixed therein light having a wavelength other than the blue-base wavelength, a green-base optical glass filter has a low transmittance, and a red-base optical glass filter gets light of an infrared range mixed therein.
The output I(xcex) of each image sensor is deteriorated due to the influence of the wavelength characteristics of the constituent members. Particularly for the blue-base light, the output is low in the sensor sensitivity, the filter transmittance and the light output, and the output of the image sensor influenced thereby is reduced compared with the red-base sensor and is most strongly affected by fluctuations in the amount of light from the lamp.
The above mentioned parameters are determined in consideration of the characteristics of the constituent members so that respective outputs of the image sensors for generating colors are as equal to one another as possible. Among the parameters, the S(xcex) and the F(xcex) do not vary once determined, while the P(xcex) to be inputted and the T fluctuate by a turn-on circuit in such a manner that the P(xcex) varies with fluctuation in a lamp-driving voltage and the T varies to a driving frequency of the image sensor.
In order to solve the above-mentioned problem that the image sensor output varies with fluctuation in a lamp-driving voltage, an invention was disclosed in Japanese Patent Laid-open No. Sho 59-53865. The invention includes image sensors having optical filters corresponding to respective wavelengths of three colors of red, blue and green, and scans one same image three times corresponding to the three color sensors. Since the sensitivities of the image sensors differ for the wavelength of a light source, the voltage of the light source is varied for each image sensor in the three-time scanning operation to control the outputs of the image sensors at a predetermined value.
FIG. 4 shows a conventional circuit using a xenon lamp as a light source. The circuit has a turn-on circuit 6 and a converting circuit 10, as described below. The turn-on circuit 6 includes an oscillator 1a, a frequency divider 2a, a waveform converting unit 3a, a lamp turn-on circuit 4 and a xenon lamp 5. In order to obtain a high power, the xenon lamp 5 is generally driven by an AC pulse voltage. The converting circuit 10, which receives light reflected at an object and converts it into an electrical signal, includes an oscillator 1b, a system control unit 7 having a microprocessor to control the whole image input device, and image sensors 8B, 8G and 8R. When the converting circuit 10 is combined with a laser device 9, the system control unit 7 is connected to the laser device 9.
The turn-on circuit 6 operates as follows. A clock having a frequency f1 is supplied from the oscillator 1a to the frequency divider 2a. The clock with the frequency f1 is frequency-divided by the frequency divider 2a and then supplied to the waveform converting unit 3a. The waveform converting unit 3a, which includes a triangular wave generator and a pulse width modulator (they are not shown), generates a conventionally-known two-phase rectangular wave and supplies it to the lamp turn-on circuit 4. The lamp turn-on circuit 4 generates a predetermined voltage with a predetermined frequency. The xenon lamp 5 is turned on at the predetermined frequency by the applied voltage.
The converting circuit 10 operates as follows. A clock CP having a frequency f2 is supplied from the oscillator 1b independent of the oscillator 1a to the system control unit 7. Then, a microprocessor (not shown) starts a predetermined operation. The system control unit 7 outputs from a terminal CPI a clock ICP to drive each image sensor and from a terminal I1 a reset signal R to read an electrical signal stored in each image sensor, and supplies the clock ICP and the reset signal R respectively to a clock terminal C and a reset terminal RE of each of the image sensors 8B, 8G and 8R, and each image sensor performs conventionally-known operation.
That is, each of the image sensors 8B, 8G and 8R has a buffer memory (not shown) for one line therein and transfers the electrical signals stored by the previous scanning to the buffer memory for one line on the basis of the signal R. Synchronizing with the scanning for next one line, each image sensor outputs the contents of the buffer memory for the one line from respective terminals IO. Signals 8BS, 8GS and 8RS of the image sensors 8B, 8G and 8R outputted from the respective terminals 10 are supplied respectively to terminals IB, IG and IR of the system control unit 7, and stored in buffer memories (not shown) located in the system control unit 7.
The signals of the image sensors stored in the buffer memories (not shown) in the system control unit 7 are transferred to the laser device 9 as necessary and outputted therefrom by a conventional laser printer.
According to the invention disclosed in the above-mentioned Japanese Patent Laid-open, the image sensor output can be inhibited from fluctuating due to variations in the lamp-driving voltage. The invention, however, cannot solve the problem that the output varies with fluctuation in the accumulating time T of light incident on each image sensor.
In the conventional example shown in FIG. 4, the turn-on circuit 6 and the converting circuit 10 have the oscillator 1a and the oscillator 1b, respectively. Accordingly, it can happen that due to the difference between the frequency of the accumulating time T of light incident on each image sensor and the turn-on frequency FL, the number of turning-on of the lamp during the light accumulating time T includes a fraction. Consequently, when the image sensors are operated at high speed, the following problem occurs. On the assumption of FL (the turn-on frequency of the xenon lamp 5)=121.44 kHz (1-pulse driving interval: 8.2345 xcexcS) and TS (scanning time for one line of each of the image sensors 8B, 8G and 8R)=3 mS, the number of pulses for turning on the lamp during the scanning time TS includes a fraction, namely, 364 or 365. In case of TS=362 xcexcS in order to operate the image sensors at high speed, the number of pulses for turning on the lamp during the scanning time TS is 43 or 44.
Accordingly, in case of FL=121.44 kHz and TS=3 mS, the variation in one pulse supplied to the xenon lamp 5 during one scanning time (TS=3 mS) of each image sensor is {fraction (1/365)} or less. However, in case of FL=121.44 kHz and TS=362 xcexcS, the variation in one pulse supplied to the xenon lamp 5 during one scanning time (TS=362 xcexcS) of each image sensor is {fraction (1/44)}. The fraction included in the number of pulses to turn on the lamp is caused by the fluctuation in the frequency of the oscillator la or fluctuation in the lamp turn-on timing. Accordingly, in case of the image sensor operating at high speed, the influence is large when the pulse to turn on the lamp is shifted by one pulse.
In order to reduce the influence due to the fluctuation in the frequency of the oscillator 1a, the lamp turn-on frequency may be increased. However, the xenon lamp and other lamps have an upper limit on the turn-on frequency. When the lamp is turned on at a frequency exceeding the upper limit, various problems are caused such as deterioration in luminance, partial turn-on, and reduction in life. Accordingly, it is difficult to increase the lamp turn-on frequency for solving the problem. As mentioned above, the accumulating time varies with the turn-on frequency of the image sensor. For example, under the above-mentioned conditions of FL=121.44 kHz (1-pulse driving interval: 8.2345 xcexcS) and TS=362 xcexcS, the variation of {fraction (1/44)} in one pulse to be supplied to the xenon lamp 5 comes out straight as an output fluctuation in the image sensor giving variation 2% or more for one pulse.
Accordingly, it is an object of the present invention to solve the above problems thereby providing an image input device which realizes a stable output even when a lamp is turned on at a high frequency affecting accumulating time of an image sensor.
In order to accomplish the above object, according to a first aspect of the present invention, an image input device comprises: a rare gas discharge lamp which sheds light on an object and is turned on by a high AC pulse voltage; and image sensors which convert an intensity of light reflected at the object into an electrical signal and are driven synchronously with the rare gas discharge lamp.
According to a second aspect of the present invention, in the first aspect, the image sensor and the rare gas discharge lamp are driven synchronously by controlling a turn-on circuit for the rare gas discharge lamp through an image transfer signal from the image sensor.